Hybrid silicon vertical cavity laser with in-plane coupling

ABSTRACT

A silicon vertical cavity laser with in-plane coupling comprises wafer bonding an active III-V semiconductor material above a grating coupler made on a silicon-on-insulator (SOI) wafer. This bonding does not require any alignment, since all silicon processing can be done before bonding, and all III-V processing can be done after bonding. The grating coupler acts to couple the vertically emitted light from the hybrid vertical cavity into a silicon waveguide formed on an SOI wafer.

FIELD OF THE INVENTION

Embodiments of the present invention are directed to vertical cavitylasers and, more particularly, to hybrid silicon/III-V vertical cavitylaser structures for use in in-plane, waveguide based photonicintegrated circuits.

BACKGROUND INFORMATION

Although silicon is an excellent material for guiding light and candetect and modulate light at high data rates, it has not yet beencapable of efficiently generating large amounts of light. Silicon may bean inefficient light emitter because of a fundamental limitation calledan indirect bandgap. An indirect bandgap prevents the atoms in siliconfrom emitting photons in large numbers when an electrical charge isapplied. Instead, silicon tends to emit heat.

One approach to generate light on a silicon material is the so-calledhybrid silicon evanescent platform. Indium Phosphide is one of a fewspecial materials, including Gallium Arsenide, which emits energy as aphoton of light when voltage is applied. Both materials may be used tomake laser diodes, and are referred to as ‘III-V materials’ on thePeriodic Table of elements because they share similar characteristics.

The hybrid silicon evanescent platform approach involves using waferbonding to fix in place a III-V semiconductor material capable ofemitting light above a silicon-on-insulator (SOI) wafer which is usedfor guiding the light. With the proper silicon waveguide design, lightthat is generated in the III-V material by current injection isautomatically coupled into the silicon waveguide and laser light isefficiently coupled to the silicon waveguides. Several examples of theselasers have been demonstrated and their merits are now well-accepted.

However, there are some inherent disadvantages of these lasers due totheir in-plane structure. To date, in-plane lasers have not demonstratedthe high wall-plug efficiencies (i.e. overall energy efficiency) ofvertical cavity surface emitting lasers (VCSELs). Vertical cavity lasersalso have inherently higher direct modulation bandwidths with lowermodulation powers. The advantages of the VCSEL result from the very lowthreshold currents that are possible for vertical cavity lasers, andthis may be due to a combination of effects including the short cavitylength and high electric field overlap with the quantum well (gain)material. If VCSELs are used, there are other problems such as alignmentissues. Current methods involve die bonding individual VCSELs abovesilicon or coupling lasers in from the edge of a silicon chip. Bothmethods are time consuming and require sensitive alignment techniques.

FIG. 1 is a graph showing simulated and experimental threshold currentfor VCSELs with different reflectivity products for the front and backmirrors. Although results may also vary depending on the exact materialused to form the laser active section, it is clear that very lowthresholds can be attained, and this has been demonstratedexperimentally. Thresholds below 1 mA are routinely obtained today.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and a better understanding of the present invention maybecome apparent from the following detailed description of arrangementsand example embodiments and the claims when read in connection with theaccompanying drawings, all forming a part of the disclosure of thisinvention. While the foregoing and following written and illustrateddisclosure focuses on disclosing arrangements and example embodiments ofthe invention, it should be clearly understood that the same is by wayof illustration and example only and the invention is not limitedthereto.

FIG. 1 is a graph showing simulated and experimental data for thresholdcurrent of VCSELs with different reflective mirror properties;

FIG. 2 is a side view of a hybrid vertial cavity laser coupled to anin-plane silicon waveguide according to one embodiment of the invention;

FIG. 3 is a graph showing normalized back reflection (on the right axis)for different chirp parameter gratings; and

FIG. 4 is a cross-sectional view of an example epistructure of a III-Vmaterial used for a laser according to one embodiment.

DETAILED DESCRIPTION

Described is a VCSEL-like laser on silicon. By combining the highefficiency and modulation bandwidth of vertical cavity lasers with thelight guiding, multiplexing, detecting, and modulating capabilities ofin-plane silicon photonics advantages may be obtained.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

Referring now to FIG. 2, embodiments of the invention involve waferbonding an active III-V semiconductor material above a grating couplermade on a silicon-on-insulator wafer. As shown an SOI wafer 200comprises a buried oxide layer 202. A passive silicon waveguide 204 isfashioned on top of the buried oxide layer 202. A metal or SIO gratingcoupler 206 is fashioned on the buried oxide layer 202 on a lateralplane with the silicon waveguide 204. According to embodiments, anactive III-V semiconductor material, forming a laser structure 208 maybe bonded above the grating coupler 206. The laser 208 comprises a III-Vactive material 210. A top mirror 212 may be a reflective metal or alsoa III-V material and may reflect greater than 99% of generated lightback into the laser cavity 214.

The laser 208 may be bonded to the substrate 200. The bonding step doesnot require any alignment, since all silicon processing can be donebefore bonding, and all III-V processing can be done after bonding,including removal of III-V material in undesired areas. The II-Vsemiconductor material may be any number of compounds, and may be chosento emit light at wavelengths ranging from 1300 nm to 1600 nm or higher.Of course other wavelengths are possible as this may also be useful inthe 980 nm range. The bonding may be performed using oxide bonding orbisbenzocyclobutene (BCB) bonding, for example. One advantage of thisdevice is that the interface thickness does not affect the lasingperformance as much as it can with traditional evanescent in-planedevice.

The vertical cavity laser 208 is then made above the grating coupler 206using standard III-V processing techniques. The top mirror 212 mayeither be formed using a metal coating reflector or a distributed Braggreflector grown as part of the III-V material epistructure. The lowermirror is formed by the grating coupler 206 itself, which can bedesigned to provide a specific reflectivity. The grating 206 sends someof vertical laser light horizontally into silicon waveguide 204 andreflects the rest back up to form the laser cavity 214. Thus, thegrating coupler 206 also acts to couple the vertically emitted lightfrom the hybrid vertical cavity 214 into a silicon waveguide 204 formedon the SOI wafer 200. The grating mirror 206 may be a high indexcontrast grating made from air on silicon or SiO₂ on silicon or a metalgrating.

The arrows 216 show the path taken by the light. Because the modaloverlap is so large and the cavity length 214 is so short, this lasermay have lower threshold, higher efficiency, lower direct modulation RFpower, and better modulation bandwidth than in-plane distributedfeedback lasers. Since it is coupled to in-plane waveguides 204, it maystill be used for on chip modulation, wavelength division multiplexing,etc.

FIG. 3 shows the reflectivity for different chirp parameters for a SOIgrating coupler. Although the maximum reflectivity shown for thisparticular design is only ˜45%, much higher reflectivities may beobtained by changing the design. The design shown in the graph waschosen to minimize reflections. Alternatively, instead of relying on thegrating coupler to provide all of the back reflection, an additionalbottom mirror may also be made by a distributed Bragg reflector in theIII-V epistructure, or by a second grating placed after the gratingcoupler in the silicon waveguide. By using different grating designs, orby quantum well intermixing, or by using different length lasercavities, this technique may be used to form arrays of lasers that canthen be wavelength division multiplexed on a single SOI chip.

FIG. 4 shows one example epistructure of the III-V material used in thelaser 208. The laser 208 may comprise of an n-contact layer 400, whichcould also include a Distributed Bragg Reflector (DBR) structure to actas one of the mirrors, an optional superlattice layer 402, which may beused protect the quantum wells from strain during bonding, a quantumwell layer 404 that generates the laser light, and a p contact layer406, which also may include a DBR structure. The silicon epistructure isa standard SOI wafer.

It should be noted that this same concept and basic structure can beused to make vertical photodetectors and vertical electroabsorptionmodulators as well. For example, in FIG. 2, the laser structure 208 mayalso be a photo-detector or modulator. For a vertical cavityelectroabsorption modulator, light would be coupled up from thewaveguides on the chip using a grating coupler into theelectroabsorption material placed above the coupler, and then coupledout of the chip to a vertically aligned fiber. The verticalphotodetector could either detect light from a vertically aligned fiberabove the chip, or detect light coupled up from the waveguide using agrating coupler.

Embodiments of the present invention are much more highly scalable thandie bonding individual VCSELs above silicon waveguides one at a time.The invention uses wafer bonding, which does not require activealignment and can be incorporated into the back end of a CMOS process.For these reasons it is highly scalable. For example, thousands oflasers may be made and placed simultaneously from one bonding step.Compared to silicon evanescent in-plane lasers this method is expectedto result in lower threshold currents, higher efficiencies, and highermodulation bandwidths. The lasers may be less sensitive to the bondingquality at the silicon/III-V interface and any fluctuations in thesilicon waveguide dimensions. In theory this technique requires lessIII-V semiconductor material than silicon evanescent lasers.

Embodiments may be used in place of silicon evanescent lasers forapplications requiring high efficiency lasers. These may include opticalinterconnects for chip-network, chip-chip, or on chip. They may becapable of direct modulation with very low RF powers compared towaveguide lasers or external silicon Mach-Zehnder modulators.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification and the claims. Rather, the scope of theinvention is to be determined entirely by the following claims, whichare to be construed in accordance with established doctrines of claiminterpretation.

1. An apparatus, comprising: a silicon on insulator (SOI) substrate; asilicon waveguide on the SOI substrate; a grating coupler fabricated onthe silicon waveguide; and a vertical III-V semiconductor device bondedto the SOI substrate above the grating coupler, wherein laser light iscoupled from the vertical III-V semiconductor device laterally by thegrating coupler into the silicon waveguide.
 2. The apparatus as recitedin claim 1 the vertical III-V semiconductor device is a laser andcomprises: an n-contact layer; a quantum well layer comprising a III-Vsemiconductor material over the n-contact layer; and a p-contact layerover the quantum well layer.
 3. The apparatus as recited in claim 2,wherein the vertical III-V semiconductor laser further comprises: asuperlattice layer between the quantum well layer and the n-contactlayer to protect the quantum well layer during bonding.
 4. The apparatusas recited in claim 2 wherein the n-contact layer comprises adistributed Bragg structure.
 5. The apparatus as recited in claim 2wherein the p-contact layer comprises a distributed Bragg structure. 6.The apparatus as recited in claim 1 wherein the vertical III-Vsemiconductor device is a laser and emits light at wavelengths greaterthan 980 nm.
 7. The apparatus as recited in claim 1 wherein the III-Vsemiconductor device is a laser bonded to the SOI substrate withbisbenzocyclobutene (BCB) bonding.
 8. The apparatus as recited in claim1 wherein the III-V semiconductor device comprises a photodetector. 9.The apparatus as recited in claim 1 wherein the III-V semiconductordevice comprises a modulator.
 10. A method comprising: forming awaveguide on a silicon on insulator (SOI) wafer; forming a gratingcoupler on the SOI wafer on a lateral plane with the waveguide; bondinga III-V semiconductor material to the SOI wafer; and fabricating a laserwith the III-V semiconductor material.
 11. The method as recited inclaim 10 wherein silicon processing is done before the bonding and III-Vsemiconductor processing is done after the bonding.
 12. The method asrecited in claim 10 wherein the bonding comprises oxide bonding.
 13. Themethod as recited in claim 10 wherein the bonding comprisesbisbenzocyclobutene (BCB) bonding.
 14. The method as recited in claim 10wherein the fabricating the laser comprises: fabricating a n-contactlayer; fabricating a quantum well layer on the n-contact layer; andfabricating a p-contact layer on the quantum well layer.
 15. The methodas recited in claim 14 wherein the n-contact layer comprises adistributed Bragg structure.
 16. The method as recited in claim 14wherein the p-contact layer comprises a distributed Bragg structure. 17.The method as recited in claim 14 wherein the fabricating the lasercomprises further comprises: fabricating a superlattice layer betweenthe quantum well layer and the n-contact layer to protect the quantumwell layer during bonding.
 18. The method as recited in claim 14 whereinthe laser emits light at wavelengths greater than 980 nm.
 19. The methodas recited in claim 14 wherein laser light is coupled from the laserlaterally by the grating coupler into the silicon waveguide.